1. Field of the Invention
The present invention relates generally to transmission lines and more particularly, to a coaxial transmission line which reduces a signal loss during transmission of a radio frequency (RF) signal, and reduces signal interference between adjacent signal lines, and is manufactured by improved packaging technologies. Further, the present invention relates to a method of manufacturing the coaxial transmission line, and a method of packaging the coaxial transmission line.
2. Description of the Related Art
Typically, a coplanar-type transmission line system and a microstrip-type transmission line system have been widely used to transmit RF signals.
The coplanar-type transmission line system is constructed so that a signal line and ground wires are arranged on the same plane. In this case, the ground wires are placed on opposite sides of the signal line. Because it is possible to arrange the signal line and the ground wires on the same plane as described above, the coplanar-type transmission line system has been widely used to make a circuit using a semiconductor substrate or a ceramic substrate.
Further, the microstrip-type transmission line system includes a signal line on an upper surface of a substrate, and a ground wire on a lower surface of the substrate. However, the microstrip-type transmission line system has a problem in that a via hole must be formed to connect the signal line provided on the upper surface of the substrate to the ground wire provided on the lower surface of the substrate. Thus, the microstrip-type transmission line system has been limitedly used for a substrate made of a soft material which allows the via hole to be easily formed, such as a printed circuit board (PCB). However, due to development of a semiconductor manufacturing technology, the microstrip-type transmission line system is also employed to the semiconductor substrate and the ceramic substrate.
Since the signal lines in the above-mentioned transmission line systems are not shielded, that is, are open, radiative signal losses of the signal lines occur during the transmission of signals, and further, signal interference occurs due to the radiative signals. In order to overcome the above problems, there is proposed a method of building a shielding wall made of a metal material to shield each signal line. However, it is difficult to execute such a method of building the shielding walls, thus undesirably causing an increase in the manufacturing costs.
Recently, as a frequency in RF electrical equipment and an output power of the equipment becomes higher, the signal interference is further increased. Thus, those skilled in the art have made efforts to develop a coaxial transmission line which is capable of shielding the signal lines.
A conventional coaxial transmission line will be described in the following.
A conventional coaxial transmission line, which is disclosed in Korean Patent Appln. No. 10-1999-0059866 and is titled “COAXIAL TYPE SIGNAL LINE AND MANUFACTURING METHOD THEREOF”.
FIG. 1 is a sectional view to show the conventional coaxial transmission line. In order to provide dielectric layers between signal lines and ground layers during the manufacture of the coaxial transmission line, a liquid polymer material 3 is coated on a semiconductor substrate 1 through a spin coating method, and then the semiconductor substrate 1 coated with the liquid polymer material 3 is cured at 200° C. While the above-mentioned process is repeated, a flattening process is carried out, thus providing a multi-layer film. Metal lines for signal lines 4 are arranged on the film, and another polymer material 5 is coated on the metal lines, and then cured.
The method of manufacturing the coaxial transmission line will be described in detail in the following. First, grooves are formed on predetermined portions of the semiconductor substrate 1, and a first ground layer 2 is formed on the substrate 1. Subsequently, the liquid polymer material 3 is coated on the first ground layer 2, and then flattening and curing processes are executed. The signal lines 4 are arranged on the cured polymer material 3, and a second polymer material 5 is coated on the signal lines 4. Thereafter, the second polymer material 5 is cured and etched, thus providing a second ground layer 6.
However, such a method has problems in that it takes a longer time to manufacture the coaxial transmission line, and a stress is generated due to a difference in thermal expansive coefficient of interface between the polymer film and other material during the hot curing process, thus reducing yield. Meanwhile, in order to reduce a signal loss caused in a dielectric during a signal transmission, an air layer whose dielectric loss is little may be used in place of the dielectric. However, the method of using the air layer has a problem in that it is difficult to manufacture the coaxial transmission line, thus incurring great expense.